Theodoric of Ringelheim - Sources?

What evidence that Reginhart, count in West Saxony is the father of Theodoric of Ringelheim?

Medlands finds none:
1. --- (-before 908). The name of the husband of Mathilde is not known but, if the chronicler Widukind is to be believed, he was a descendant of Duke Widukind[43]. This appears corroborated by the transmission of the name Widukind to his grandson. If this is correct, from a chronological point of view it is likely that he was the great-grandson of Widukind. m MATHILDE, daughter of --- (-911 or after). The mother of "Thietrici comitis" is named "abbatissa Mahthild" in the Vita Mathildis Reginæ, which specifies that she was abbess at Herford[44]. According to Grote[45], she was recorded as abbess in 908 and 911 but the primary sources on which this is based have not so far been identified. Mathilde & her husband had one child:
a) THEODERIC (-8 Nov 917[46]).

http://fmg.ac/Projects/MedLands/SAXONY.htm#_Toc484586536