Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Allah Almighty, on the tongue of your Prophet (peace and blessings be upon him), made obligatory upon you four rak'ahs when resident, two rak'ahs in travel, and one rak'ah in fear.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6054