Sayyiduna Imran bin Husain (may Allah be pleased with him) narrates that I had a tumor (cancer), so I asked the Noble Prophet (peace and blessings be upon him) about (performing) prayer. He said: Perform prayer standing; if you are not able, then perform it sitting; if you are not able to do even that, then perform it lying down.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصلاة / 231