سنن النسائي

It was narrated from Sa'd bin Hisham that Aishah said: "We used to prepare siwak and water for wudu for the Messenger of Allah (ﷺ). Allah (SWT) would wake him when He willed to wake him at night, then he would clean his teeth and make wudu, and pray nine rak'ahs, not sitting until the eighth, when he would praise Allah (SWT) and send blessings upon His Prophet and supplicate between them, but he did not say the taslim. Then he prayed the ninth and sat, and said something similar, praising Allah (SWT) and sending blessings upon His Prophet (ﷺ), then he said a taslim that we could hear, then he prayed two rak'ahs sitting down."