Hadith.1340 - I heard our teacher, Muhammad ibn al-Hasan, say: It was narrated that Imam Jafar ibn Muhammad Al-Sadiq (as) was asked about the words of Allah (swt), the Mighty and Majestic:
"And when you travel throughout the land, there is no blame upon you for shortening the prayer, if you fear that those who disbelieve may harm you" (Surah An-Nisa 4:101).
Imam (as) said
"This refers to a second form of shortening, where a man reduces two rak'ahs to one."