Sunan Abu Dawud
952
Sahih+3

‘Imran b. Husain said :I had a fistula; so I asked the prophet (ﷺ). He said: offer prayer in standing condition; if you are unable to do so, then in sitting condition: if you are then at your side(i.e, in lying condition)

SahihAl-Albani
SahihMuhammad Muhyi Al-Din Abdul Hamid