Praise be to Allah.
Shaykh Ibn Baaz (may Allaah have mercy on him) said:
The majority of scholars said that one tasleem is sufficient because evidence to that effect was narrated in some ahaadeeth. But a group of scholars stated that there have to be two tasleems because of the soundness of the ahaadeeth from the Prophet (peace and blessings of Allaah be upon him) which prove that, and because he said: “Pray as you have seen me praying.” (Narrated by al-Bukhaari in his Saheeh). This view is the one which is correct.
The view that one tasleem is sufficient is da’eef (weak) because of the weakness of the ahaadeeth that were narrated concerning that, and because there is ambiguity in these reports. If they are saheeh then they are shaadhdh (odd), because they go against reports which are more saheeh, more sound and more clear. But whoever does that – i.e., says one tasleem – out of ignorance or believing that the ahaadeeth concerning that are saheeh, his prayer is still valid. And Allaah is the Source of strength.