Mustadrak al-Hakim — Hadith 575

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) was asked about the circumstances of the expedition of Tabuk (the Hour of Hardship), and he said: We set out for Tabuk in intense heat. At one place we were struck by such severe thirst that we felt our necks would be severed, to the extent that a man would slaughter his camel, squeeze its stomach contents and drink (the fluid), and place the rest over his liver. Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) said: O Messenger of Allah! Allah Almighty has always placed good in your supplications, so supplicate for us. He (peace and blessings be upon him) asked: "Would you like that?" He said: Yes. So he (peace and blessings be upon him) raised his hands, and he had not yet lowered them when clouds covered the sky and such rain fell that everyone filled their vessels. Then we looked and saw that the rain had not gone beyond the bounds of the army.
This hadith is sahih (authentic) on the condition of Imam Bukhari and Imam Muslim, and it contains this point of fiqh: the fluid of the stomach contents of a lawful animal does not make water impure.