Siman 397
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מִתְאַבְּלִין עַל פִּי עֵד אֶחָד, וְעֵד מִפִּי עֵד וְעוֹבֵד כּוֹכָבִים מֵסִיחַ לְפִי תֻּמּוֹ.
1 One observes mourning rites on the testimony of a single witness or [on the testimony of] a witness testifying to what he has heard from an [eye-] witness or if a Gentile makes an incidental statement [regarding] one's death.
ב שְׁנַיִם אוֹמְרִים: מֵת, וּשְׁנַיִם אוֹמְרִים: לֹא מֵת, אֵינוֹ מִתְאַבֵּל.
2 [If] two [witnesses] say, 'He died,' and two [other witnesses say, 'He did not die,' the [near-of-kin] does not observe mourning rites.