He used to pray the night prayer in the mosque of the Messenger of Allah ﷺ, then perform witr with a single rakah, adding nothing more.Someone said to him, "Do you pray witr with just one rakah and add nothing more, Abu Ishaq?"He replied, "Yes. I heard the Messenger of Allah ﷺ say, 'The one who does not sleep until he has prayed witr is prudent.'"
أنه كان يصلي العشاء الآخرة في مسجد رسول الله ﷺ ثم يوتر بواحدة، لا يزيد عليها، قال: فيقال له: اتوتر بواحدة، لا تزيد عليها يا أبا إسحاق؟ فيقول: نعم، إني سمعت رسول الله ﷺ يقول: الذي لا ينام حتى يوتر حازم.
Ahmad (1461) narrated it from Yaqub, who said, my father told me, from Ibn Ishaq, Muhammad bin Abdul-Rahman bin Abdullah bin Husayn told me, that he narrated from Saad bin Abu Waqqas, and he related it.Its isnad is hasan because of Ibn Ishaq and his teacher Muhammad bin Abdul-Rahman bin Husayn, and even though no one narrates from him except Muhammad bin Ishaq, he said, "He was one who fasted much and stood much in prayer," and had he known any fault in him he would have made it clear. That is why Ibn Hibban mentioned him in Thiqat (7/413).What draws attention is that Saad bin Abu Waqqas was asked about Witr with a single unit, and he was content to answer simply, "Yes," then he began to relate about someone who prays the last night prayer and then performs Witr before sleeping, saying that he is resolute. This has a corroborating narration as mentioned before.On this subject there is also a report from Abu Hurairah, narrated by Bazzar in Kashf Astar (736), and by Tabarani in Awsat (5059), which contains Sulayman bin Dawud Yamami, who is daif. Bazzar said, "Sulayman bin Dawud is not corroborated in his hadith, and he is not strong, and his hadiths point to his weakness." End of quote. On this basis Haythami also judged him daif in his Majma (2/245).And from Uqba bin Amir, narrated by Tabarani in Kabir (30/17/303), which contains Ibn Lahia, and on this basis Haythami also faulted it in his Majma.