Praise be to Allah.
Reciting a soorah after al-Faatihah in the first two rak‘ahs of the prayer is mustahabb (recommended), not waajib (obligatory), according to the majority of scholars.
Abu Hurayrah (may Allah be pleased with him) said: In every prayer there is recitation, so what the Prophet (blessings and peace of Allah be upon him) made us hear we make you hear, and what he recited quietly we recite quietly. Whoever recites the Essence of the Book (i.e., al-Faatihah), that will suffice him, but whoever does more than that, that is better.
Narrated by al-Bukhaari (738) and Muslim (396).
An-Nawawi (may Allah have mercy on him) said: The words “Whoever recites the Essence of the Book (i.e., al-Faatihah), that will suffice him, but whoever does more than that, that is better” indicate that it is obligatory to recite al-Faatihah and that the prayer is not valid without it.
It also indicates that reciting a soorah after it is mustahabb. There is consensus on that with regard to Fajr, Jumu‘ah and the first two rak‘ahs of every prayer, and it is Sunnah according to all scholars. Al-Qaadi ‘Iyaad (may Allah have mercy on him) narrated from some of the companions of Maalik that it is obligatory to recite a soorah after al-Faatihah, but this is an odd view and is to be rejected.
End quote from Sharh Muslim (4/105).
The minimum that may be recited is one verse; Ahmad (may Allah have mercy on him) recommended that it should be a lengthy verse, such as the verse on debt (al-Baqarah 2:282) or Aayat al-Kursi (al-Baqarah 2:255).
It says in Sharh al-Muntaha (1/191): al-Qaadi (Abu Ya‘la) and others said: It is sufficient to recite one verse, except that Ahmad recommended that it should be a lengthy verse, such as the verse on debt (al-Baqarah 2:282) or Aayat al-Kursi (al-Baqarah 2:255). End quote.
If the worshipper decides to recite only one verse, it should be a verse that conveys a complete meaning or a ruling when recited on its own.
Al-Bahooti (may Allah have mercy on him) said in Kashshaaf al-Qinaa‘ (1/342): What appears to be the case is that it is not sufficient to recite a verse that does not convey a meaning or ruling when recited on its own, such as (interpretation of the meaning)
“Thumma nazara (Then he considered [again])”
“Mudhaammataan (Dark green [in color])”
And Allah knows best.