this couplet is quite straightforward. here zarb-e-yadullahi is used as a metaphor for holding fast unto the prophet's sunnah. sallallahu `alayhi wa sallam. this is derived from the verse: yadullaha fawqa aydihim / Allah aids those hands [which have sworn fealty upon the prophets] the hands here are of those companions who did bay'ah or swore fealty to RasulAllah. and a tangential reference that by doing so, they promised to fight - whether their own nafs or the enemy on the battlefield. and sajdah-e-shabbiri is a metaphor for sacrifice, martyrdom or intense worship. because sayyiduna Husayn raDiyallahu anhu, also known as shabbir was martyred when he bowed down to prostrate - and even in karbala, in such intense trial, he did not forsake prayer. so the couplet means: there are only two gems for a muslim that of following the prophet and worshipping Allah at all times or there are only two gems for a muslim the enthusiasm to strive and the spirit to sacrifice. fa'afham wAllahu ta'ala a'alam.
:s1: Allama Muhammad Iqbal wrote: Islam kay daaman mein bas do hii tau gauhar hain Ik zarb-e-Yadullahi (?), ik sajdah -e Shabbiri! I am not sure if the word is Yadullahi or Asad-ilaahi--the rest I am sure of. Someone please help! In any case the translation becomes: In the bosom of Islam there are only two pearls: The blow of The Lion of God and the prostration of Hussain!