St. Alphonsus attests to the fact how the Holy Monk St. Goerge of San Saba openly confessed to the Mohammedans: "But the holy monk (St. George of San Saba) having declared that Mahomet was
a disciple of the devil, and that his followers were in a state of perdition, he also was condemned (to martyrdom) with his companions."
What did St. Peter Mavimenus tell the Mohammedans? Did he say, "We worship the same God, all is well" No! He told them the truth, he put it this way to them "Whoever does not embrace the Catholic Christian religion will be damned,
as was your false prophet Mohammed."
Just a couple of examples on what the Saints have said regarding Mohammedans.
Also, Pius IX in the syllabus of errors, points to these two specifically:
"Every man is free to embrace and profess that religion which, guided by the light of reason, he shall consider true." (Proposition XV).
"Man may, in the observance of any religion whatever, find the way of eternal salvation and arrive at eternal salvation." (Proposition XVI).
"Those who have learned theology well," says St. Basil, “
will not allow even one iota of Catholic dogmas to be betrayed. They will, if necessary, willingly undergo any kind of death in their defense."
St. Peter Canisius puts it this way: "Who is to be called a Christian? He who confesses the doctrine of Christ and His Church.
Hence, he is truly a Christian thoroughly condemns and detests, the Jєωιѕн, Mohammedan, and the heretical cults and sects."
http://www.catholicapologetics.info/apologetics/islam/worship.htm