It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that in the beginning, the Prophet (peace and blessings be upon him) used to offer eighteen rak‘ahs at night, three rak‘ahs of witr, and two rak‘ahs (of the sunnah of Fajr). But when the Prophet (peace and blessings be upon him) grew older, their number was reduced to nine, six, and three.
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2714
Hadith Gradingحکم دارالسلام:صحيح، وقد أضطرب فيه على يحيى بن الجزار، فروي عنه عن ابن عباس، ومرة عن أم سلمة، ومرة عن عائشة