The correct opinion is that Salatul Tasbih is not sunnah (from the prophetic teachings), and that the hadith about it is weak for several reasons which are detailed below.

There is no basis for the false superstition about marriage in Shawwal. It is one of the leftovers of the Jahiliyyah (pre-Islamic era) when they used to be superstitious about that because of the connotations of the name of Shawwal which related to the drying up of the camel’s milk and the lifting of its tail.

Takbir in unison before ‘Eid prayer is an innovation because there is no evidence for it, and the Prophet (peace and blessings of Allah be upon him) said: “Whoever does an action that is not part of this matter of ours will have it rejected.” What `Umar (may Allah be pleased with him) did is not evidence for reciting takbir in unison; rather what `Umar (may Allah be pleased with him) did was to say takbir by himself, and when the people heard him they said takbir, each one by himself. This does not indicate that they all said takbir in unison.