the prophet ( pbuh) proceeded direct to the house of abu bakr who, immediately accompanied him and both set out southwards, clambered up the lofty peak of mountain thawr, and decided to take refuge in a cave.[ibn hisham 1/483; za'd al-ma'ad 2/52]
the assassins who laid siege to the house were waiting for the zero hour when someone came and informed them that the prophet [pbuh] had already left. they rushed in and to their utter surprise, found that the person lying in the prophet’s bed was ‘ali not muhammad [pbuh]. this created a stir in the whole town. the prophet [pbuh] had thus left his house on safar 27th, the fourteenth year of prophethood, i.e. 12/13 september 622 a.d.[rahmat-al-lil'alameen 1/95]
knowing already that quraish would mobilize all its potentials to find him, he played a clever trick on them and instead of taking the road to madinah in north side of makkah as the polythiest would expect, he walked along a road least expected lying south of makkah and leading to yemen. he walked for 5 miles until he reached a rough rocky mountain called thawr. there his shoes were worn out, some said he used to walk tiptoe in order not to leave a trail behind him. abu bakr [r] carried him up the mountain to a cave called after the name of the mountain, cave thawr.