Musnad Ahmad

Awam bin Hawshab (may Allah have mercy on him) says that I asked Mujahid (may Allah have mercy on him) about the prostration (sajdah) in Surah Sad. He replied, "Yes! I asked this issue to Sayyiduna Ibn Abbas (may Allah be pleased with them), and he answered, 'Do you recite this verse: «﴿وَمِنْ ذُرِّيَّتِهِ دَاوُودَ وَسُلَيْمَانَ . . . . .﴾ [الأنعام : 84]» "And among their descendants were Dawood and Sulaiman . . . . ." and at its end it is: «﴿فَبِهُدَاهُمُ اقْتَدِهْ﴾ [الأنعام :90]» "So follow their guidance." It is as if your Prophet (peace and blessings be upon him) was commanded to follow the example of Dawood (peace be upon him) (and he performed prostration, therefore, in his following, prostration should be performed at the verse of prostration in Surah Sad).