ENOfficial
It was narrated that Ibn ‘Abbas said: “It was said: ‘O Messenger of Allah, why did you support (by supplicating for) those who shave (their heads) three times and those who cut (their hair) only once?’ He said: ‘Because they did not entertain any doubts.’”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا يُونُسُ بْنُ بُكَيْرٍ، حَدَّثَنَا ابْنُ إِسْحَاقَ، حَدَّثَنَا ابْنُ أَبِي نَجِيحٍ، عَنْ مُجَاهِدٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قِيلَ يَا رَسُولَ اللَّهِ لِمَ ظَاهَرْتَ لِلْمُحَلِّقِينَ ثَلاَثًا وَلِلْمُقَصِّرِينَ وَاحِدَةً قَالَ " إِنَّهُمْ لَمْ يَشُكُّوا " .
sunnah.com