Ammar prayed two units of prayer, and Abdul-Rahman bin Harith said to him, "Abu al-Yaqzan, I noticed you made them quite brief."Ammar replied, "Did I fall short of any of their essential requirements?"Abdul-Rahman said, "No, but you made them brief."Ammar said, "I hurried through them to ward off distraction. I heard the Messenger of Allah ﷺ say, 'A man may pray, yet perhaps nothing of his prayer is recorded for him except a tenth of it, or a ninth, or an eighth, or a seventh,'" until he reached the end of the count.
حدثنا يحيى بن سعيد عن عبيد الله قال حدثني سعيد بن أبي سعيد
أن عمارا صلى ركعتين فقال له عبد الرحمن بن الحارث يا أبا اليقظان لا أراك إلا قد خففتهما قال هل نقصت من حدودها شيئا قال لا ولكن خففتهما قال إني بادرت بهما السهو إني سمعت رسول الله ﷺ يقول إن الرجل ليصلي ولعله أن لا يكون له من صلاته إلا عشرها وتسعها أو ثمنها أو سبعها حتى انتهى إلى آخر العدد
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Its isnad is sahih. Umar bin Abu Bakr bin Abdul-Rahman bin Harith Makhzumi was deemed trustworthy by Ibn Hibban and Nasai, and Saeed bin Abu Saeed is Maqburi, the well-known trustworthy narrator, and Ubaydullah is Ibn Umar Umari, also the well-known trustworthy narrator. The hadith was narrated by Ibn Mubarak in Zuhd 459, and Ibn Hibban 521 (Mawarid), and part of it has already appeared in similar wording at 18239 and 18240.