-اَللَّهُمَّ ا هْدِنِيْ وَ سَدِّدْنِيً
Allaahummah-dinii wa saddidnii. (‘Ali ibn Abi Talib. Sahih Muslim #4904)
O Allah, give me guidance and keep me firm (on the path of guidance).
This du’aa, with the addition of “وَقِنِيْشَرَّنَفْسِيْ” is also the short du’aa of istikharah that one should make when faced with quick decision-making and there is no time for Istikharah salaat.
(Source: Khalid Baig’s commentary to du’aa #50 in the Accepted Whispers: Munajat-e-Maqbul)