This is based upon a description in 'Gosanjoin Gosokuiki' (record of the enthronement of Emperor Gosanjo) written by OE no Masafusa that, after his enthronement, Emperor Gosanjo did not hold shaku (a mace) and showed mudra of Dainichi Nyorai.
これは大江匡房が著した「後三条院御即位記」に、即位時、後三条天皇は笏を持たず、手で大日如来の印相をしていたと書かれていることによる。