Question about Dua, re: "sadqe tufail"?

[BillieAli]

ASWB,
Learned brothers and sister I need your help.
Could anyone advise if it is incorrect to make Dua using "sadqe tufail"?
Ive tried to search on google for answers but was unsuccessful.
There are many schools of thought but i wanted to know if anyone could back up with examples of why it is or is not incorrect?
"sadqe tufail" from my humble understanding roughly translates to "through the grace of the rasool please accept my dua".

Also i need help about a story in a hadith that i was trying to remember but can not find.
It is a story about the prophet sending his sahiba's from one town to another and telling them not to pray Asr till they get there. And how the Sahiba's have an argument at the time of Asr as they had not reached their destination and half prayed at the correct time and half travelled on to their destination and prayed when they got there. Upon returning the prophet says that both groups were correct as they used their best reasoning to make their decisions.
Does anyone have the story as it is supposed to be told?

[Saad]

Tawassul and it's position in Islam

http://www.central-mosque.com/aqeedah/taw1.htm

And the incident you have mentioned can be found in The Differences of the Imams.