Sahih Al Bukhari Hadith Number 835

Narrated `Abdullah: When we prayed with the Prophet (peace be upon him) said, “Don’t say As-Salam be on Allah, for He Himself is As-Salam, but say, at-tahiyatu li l-lahi wa s-salawatu wa t-taiyibat. As-salamu `alaika aiyuha n-Nabiyu wa rahmatu l-lahi wa barakatuh. As-salamu `alaina wa `ala `ibadi l-lahi s-salihin. (If you say this then it will reach all the slaves in heaven or between heaven and earth). Ash-hadu al la-ilaha illa l-lah, wa ash-hadu anna Muhammadan `Abduhu wa Rasuluh.’ Then select the invocation you like best and recite it.” (See Hadith No. 794, 795 & 796).

10  Call to Prayers (Adhaan)
Hadith number 835
Volume no 1
Chapter: 150 ( What optional invocation may be selected after the Tashah-hud, and it is not obligatory.)