Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "Whatever I forbid you, avoid it, and whatever I command you, do of it as much as you are able. Indeed, those before you were destroyed only because of their many questions and their disagreements with their prophets."
(b) The position of Sayyiduna Umar and Ibn Mas'ud (may Allah be pleased with them both) is that one who finds neither water nor earth should not pray; in their view the only means of purification for a junub person is water, and when someone does not find water, in their view he should not pray.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1029