Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that Allah Almighty has, through the tongue of your Prophet (peace be upon him), made obligatory four rak‘ahs of prayer in residence, two rak‘ahs in travel, and one rak‘ah in fear.
Hadith Referenceصحيح ابن خزيمه / كتاب: الصلاة / 304