“Soon shall We cast terror into the hearts of the Unbelievers, for that they joined companions with Allah, for which He had sent no authority”. – Qur’an 3:151
Quran Commentary on 3:151
“We will cast terror (read ru‘b or ru‘ub) into the hearts of the disbelievers: after departing from Uhud they resolved to return in order to exterminate the Muslims, but they were terrified and did not return; for what they have associated, because of their associating, with God that for which He has revealed no warrant, that is, [no] argument in support of its worship, namely, idols; their abode shall be the Fire; evil is the abode, the resting place, of the evildoers, the disbelievers.” 
Asbab Al-Nuzul by Al-Wahidi
“(We shall cast terror into the hearts of those who disbelieve) [3:151]. Said al-Suddi: “After Uhud, Abu Sufyan and the idolaters headed toward Mecca. But after they traversed a certain distance, they felt regret, saying: ‘Evil is that which we have done!
We massacred them [Muslims] such that none of them remained except those who fled from the battlefield and then we left them alone. Go back and exterminate them’. But when they decided to do so, Allah, exalted is He, cast terror in their hearts which made them decide against what they had resolved to do. Allah, exalted is He, then revealed this verse.” 
Quran Translation and Commentary by Maulana Mufti Mohammad Shafi
“The promise of casting awe and fear into the hearts of the disbelievers in this verse was made in the background of the battle of Uhud when the disbelievers of Arabia marched back to Makkah without any obvious reason and inspite of defeat overtaking Muslim (Baydawi). However, after having covered a certain distance on their way to Makkah, they awoke to their folly.
When they thought of marching back to Madinah, Allah Almighty filled their hearts with such awe and fear that they could not muster the courage to do so. The most they could do was to hire a Madinah-bound villager to go there and tell Muslims that theu were coming back.
But, this whole deal came into the knowledge of the oly Prophet (p) in Madinah through revelation. He marched to Hamra al-Asad to apprehend them but they had already run away from there. This was the background in which the present verse was revealed.” 
 Tafsir al-Jalalayn http://www.altafsir.com/Tafasir.asp?tMadhNo=0&tTafsirNo=74&tSoraNo=3&tAyahNo=151&tDisplay=yes&UserProfile=0&LanguageId=2
 Asbab Al-Nuzul by Al-Wahidi http://www.altafsir.com/Tafasir.asp?tMadhNo=0&tTafsirNo=86&tSoraNo=3&tAyahNo=151&tDisplay=yes&UserProfile=0&LanguageId=2
 Quran Translation and Commentary by Maulana Mufti Mohammad Shafi