Someone asked, "Messenger of Allah, why did you invoke blessings three times for those who shaved their heads, but only once for those who shortened their hair?"He replied, "Because they did not have any doubt."
حدثنا محمد بن عبد الله بن نمير حدثنا يونس بن بكير حدثنا ابن إسحق حدثنا ابن أبي نجيح عن مجاهد
قيل يا رسول الله لم ظاهرت للمحلقين ثلاثا وللمقصرين واحدة قال إنهم لم يشكوا
No references are available.
Sourcing, its chain of transmission is daif. Ibn Abu Shayba narrated it in 14/453 from the hadith of Ibn Ishaq through this route, and Busiri graded it sahih. Ibn Abu Najeeh is a mudallis, hadith no. 378, and I did not find an explicit statement of his having heard it directly.