Sunan Abi Dawud Book 5, Hadith 74
The Messenger of Allah (ﷺ) as saying: When one of you gets up at night, he should begin the prayer with two short rak'ahs.
They (the people) used to pray between the Maghrib and 'Isha. The version of Yahya adds: The verse tatajafa junubuhum also means so.
He should then prolong it afterwards as much as he likes.
Abu Dawud said: This tradition has been transmitted by Hammad b. Salamah, Zuhair b. Mu'awiyah and a group of narrators from Hisham. They transmitted it as a statement of Abu Hurairah himself (mauquf).
This tradition has also been transmitted by Ibn 'Awn from Muhammad (b. Sirin). This version has the wordings: These two rak'ahs were short.