Sahih Muslim Book 4, Hadith Number 1625.

Chapter : Permissibility of observing Nafl (voluntary prayer) standing or sitting and observing some part of it standing or sitting.

Sa’d b. Hisham reported: I went to ‘Abdullah b. ‘Abbas and asked him about the Witr prayer, and the rest of the Hadith is the same as recorded in this event. She (Hadrat ‘Aisha) said: Who is that Hisham? I said: Son of ‘Amir. She said: What a fine man ‘Amir was ! He died as a martyr in the Battle of Uhud.