Praise be to Allah.
Giving the title Sayyiduna to the Prophet (blessings and peace of Allah be upon him) is right and proper, because he is the master (sayyid) of the sons of Adam, and it is proven that he (blessings and peace of Allah be upon him) said: “I am the master (sayyid) of the sons of Adam and no boast.” He is the master of mankind and the Messengers. So if a man says Sayyiduna Muhammad, and: Allahumma salli ‘ala Sayyiduna Muhammad (O Allah, send blessings upon our master Muhammad), there is nothing wrong with that. He is the master (Sayyid) of the sons of Adam and the master of mankind.
He only disliked that from people during his lifetime because he feared that they might exaggerate. So when they said, You are our master (sayyid), he said: “Al-Sayyid is Allah, may He be blessed and exalted,” so as to block the means (that might lead to exaggeration), as he was afraid that they might exaggerate about him (blessings and peace of Allah be upon him). But after he died (blessings and peace of Allah be upon him), and as he told us that he is the master (sayyid) of the sons of Adam, there is nothing wrong with saying Sayyiduna (blessings and peace of Allah be upon him). So he is the best of us, and our master (sayyid), and our leader (imam), and he is a close friend of the Most Merciful (blessings and peace of Allah be upon him).
With regard to other people, it is better not to say Sayyiduna or to address them as such, but if it is said that so and so is their sayyid, meaning their leader or ruler or the head of the tribe, there is nothing wrong with that. It is like what the Prophet (blessings and peace of Allah be upon him) said to his grandson al-Hasan: “This son of mine is a sayyid (master)” and he said: “Who is the sayyid of such and such a tribe?” And he said to the Companions on the day Sa‘d ibn Mu ‘aadh came to pass judgement on Banu Qurayzah: “Stand up to greet your sayyid.” So there is nothing wrong with it.
But one should not say “Ya Sayyiduna (O our master)”; it is better not to say it because the Messenger (blessings and peace of Allah be upon him) said: “Al-Sayyid is Allah, may He be blessed and exalted,” and because this may lead to arrogance or exaggeration if it is said to another human being.
So it is better to address people as “our brother” or “Abu So and so”, and this is sufficient. But there is nothing wrong with saying it in some contexts, such as the following: saying “Who is the sayyid of such and such a tribe?”; or “This (person) is a sayyid” because he is descended from the family of the Prophet, or because he is a faqeeh or a scholar or noble in and of himself and in his attitude and actions, or because he is generous -- in such cases, there is nothing wrong with it. But if there is the fear that calling him sayyid may make him become big-headed or think too highly of himself, then it should not be done.
This word should not be used for a kaafir or hypocrite or sinner; rather it should only be said to a senior Shaykh or a leading faqeeh or scholar who is known to be righteous, good, forbearing, virtuous and generous.
If such people are called sayyid, there is nothing wrong with it, but with regard to addressing them as “Ya Sayyiduna”, it is better not to do that. But if an immoral person or hypocrite is the leader of his people, there is nothing wrong with addressing him by saying “Ya sayyid Bani Fulaan (O leader of the tribe of So and so”).
With regard to the second part of the question, the names of the Lord, may He be exalted, are of two types:
Those which it is permitted to call people by and those which it is not permitted to call people by. So one cannot be called Khallaaq (Creator) or Razzaaq (Provider) or Rabb al-‘Aalameen (Lord of the Worlds), but there is nothing wrong with names such as ‘Azeez (strong) or Baseer (seeing). Allah called some of His creation by such names, for example (interpretation of the meanings):
“so We made him [man] hearer and seer”
“The wife of Al‑‘Azeez said…”
These names may be given to created beings, e.g. ‘azeez qawmihi (leader of his people), baseer bi’l-umoor (having insight into things), baseer (referring to one who can see, as opposed to one who is blind), and hakeem meaning wise. There is nothing wrong with these names, because they are common between Allah and created beings.
But Allah has the most perfect level of these attributes. But names that can only belong to Allah should not be given to anyone other than Allah, so a human being cannot be called Allah or al-Rahmaan (the Most Merciful) or al-Khallaaq (the Creator) or al-Razzaaq (the Provider) or Khaaliq al-Khalq (the creator of creation) or other similar names that can only be given to Allah, may He be glorified and exalted. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).