Sunan an-Nasa'i 2300
سنن النسائي#2300SahihIt was narrated that Hamzah bin 'Amr said:"I used to fast continually at the time of the Messenger of Allah. I said: 'O continually while traveling.' He said: 'If you wish then fast, and if you wish then do not fast
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiHasan
In-book reference: 22:211