On the authority of Abu Abbas Abdullah bin Abbas (may Allah be pleased with him) who said:

One day I was behind the Prophet (peace and blessings of Allah be upon him) [riding on the same mount] and he said, “O young man, I shall teach you some words [of advice]: Be mindful of Allah and Allah will protect you. Be mindful of Allah and you will find Him in front of you. If you ask, then ask Allah [alone]; and if you seek help, then seek help from Allah [alone]. And know that if the nation were to gather together to benefit you with anything, they would not benefit you except with what Allah had already prescribed for you. And if they were to gather together to harm you with anything, they would not harm you except with what Allah had already prescribed against you. The pens have been lifted and the pages have dried.” It was related by at-Tirmidhi, who said it was a good and sound hadeeth. Another narration, other than that of Tirmidhi, reads: Be mindful of Allah, and you will find Him in front of you. Recognize and acknowledge Allah in times of ease and prosperity, and He will remember you in times of adversity. And know that what has passed you by [and you have failed to attain] was not going to befall you, and what has befallen you was not going to pass you by. And know that victory comes with patience, relief with affliction, and hardship with ease.

عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: "كُنْت خَلْفَ رَسُولِ اللَّهِ صلى الله عليه و سلم يَوْمًا، فَقَالَ: يَا غُلَامِ! إنِّي أُعَلِّمُك كَلِمَاتٍ: احْفَظْ اللَّهَ يَحْفَظْك، احْفَظْ اللَّهَ تَجِدْهُ تُجَاهَك، إذَا سَأَلْت فَاسْأَلْ اللَّهَ، وَإِذَا اسْتَعَنْت فَاسْتَعِنْ بِاَللَّهِ، وَاعْلَمْ أَنَّ الْأُمَّةَ لَوْ اجْتَمَعَتْ عَلَى أَنْ يَنْفَعُوك بِشَيْءٍ لَمْ يَنْفَعُوك إلَّا بِشَيْءٍ قَدْ كَتَبَهُ اللَّهُ لَك، وَإِنْ اجْتَمَعُوا عَلَى أَنْ يَضُرُّوك بِشَيْءٍ لَمْ يَضُرُّوك إلَّا بِشَيْءٍ قَدْ كَتَبَهُ اللَّهُ عَلَيْك؛ رُفِعَتْ الْأَقْلَامُ، وَجَفَّتْ الصُّحُفُ" . رَوَاهُ التِّرْمِذِيُّ [رقم:2516] وَقَالَ: حَدِيثٌ حَسَنٌ صَحِيحٌ. 
وَفِي رِوَايَةِ غَيْرِ التِّرْمِذِيِّ: "احْفَظْ اللَّهَ تَجِدْهُ أمامك، تَعَرَّفْ إلَى اللَّهِ فِي الرَّخَاءِ يَعْرِفُك فِي الشِّدَّةِ، وَاعْلَمْ أَنَّ مَا أَخْطَأَك لَمْ يَكُنْ لِيُصِيبَك، وَمَا أَصَابَك لَمْ يَكُنْ لِيُخْطِئَك، وَاعْلَمْ أَنَّ النَّصْرَ مَعَ الصَّبْرِ، وَأَنْ الْفَرَجَ مَعَ الْكَرْبِ، وَأَنَّ مَعَ الْعُسْرِ يُسْرًا".

Reference : 40 Hadith Nawawi 19
English translation : Hadith 19
Link: http://sunnah.com/nawawi40/19