You should try your best to write the full phrase. Many scholars have said you should write it in its entirety. Other scholars say it is acceptable to abbreviate.
Shaykh Yahya is more conservative when it comes to this issue. Out of respect and love for our beloved Prophet Muhammad صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ we should mention salutations upon him in their entirety.
Shaykh Yahya Ibrahim answers and explains..