Whoever held tight my Sunnah (the way of Prophet Muhammad) at the time of sedition in my Community, he will get reward equivalent to a hundred Martyrs. Is it a hadith, and if it is how should we comprehend it?

Yes, it is. (1)
Whenever the depravities and new elements (of life) outside Islam overrun the Islamic society; and exercise its authority over society, the one who held tight to Sunnah will get reward equivalent to a hundred Martyrs. It is indeed, the truths of Quran and the principles of Sunnah are inseparable. It is so risky and dangerous, at the time, when new elements of life outside Islam are in vogue in Islamic community and when seditious acts are carried out by the majority of the society. We acquire more merit in Gods sight as much as our actions get harder (at dangerous times when new elements outside Islam enmesh the society). In those dangerous times, how great duty is that, to serve and to enlighten the community at first about the truths of faith and then about the meanings of Quranic decrees and finally the importance of following the Sunnah. It requires even more self-sacrificing than that of a Martyrs so that it can get the reward equivalent to many Martyrs. While the one who dies as a Martyr through the way of God shows an instantaneous self-sacrificing, the one who is in the same state of mind who serves to the truths of faith, Quran, and Sunnah shows self-sacrificing behavior every single day of his life.

Therefore, as much as times get brutal and seditions rise the merits of our actions rise too. In addition, there is a great encouragement in this hadith for people to follow Sunnah.

The word Tamassuq (to hold tight to Sunnah) in the hadith includes the following meanings (2):

1. Tamassuq is determination, patience, and insistence.
2. Tamassuq requires continuity.
3. Tamassuq is to maintain the whole.
4. Tamassuq reflects the struggle between opposite powers.


Sources
1. Baihaqi quoted in Mishkaat
2. Kamûsul-Muhit III, 329; el-Mucemul-Vasit s. 869; el-Mufredat, s,469